A DC-AC converter of this type is known from U.S. Pat. No. 4,415,838 and U.S. Pat. No. 4,748,383. In this known converter a transformer is present in the load circuit (in which the lamp is incorporated). This transformer has two secondary windings which form part of the control circuits for the semiconductor switching elements. The switching elements are rendered alternatively conducting and non-conducting by means of the transformer and the control circuits respectively.
However, some basic short comings of the known circuit are that the circuit is started by applying a relatively large transient current to the load circuit and the resonant circuit so that the resonant circuit begins to oscillate to provide the drive signal for the semiconductor switching elements. The oscillation frequency of the resonant circuit is controlled by the load circuit. This results in a poor switch time for the semiconductor switching elements and poor soft starting for ignition of the lamp. It is also known that this circuit is unable to provide the dimming function for controlling the lamp brightness.
It is an object of the invention to overcome the above-mentioned problems by providing a circuit using an IR2155 self-oscillating half-bridge driver in which the pulse width of the drive signal for the semiconductor switching elements can be modulated during the ignition and operation of a gas discharge lamp so as to provide preheating and dimming functions for the electronic ballast, and to provide a circuit which can be used for output over-current protection.
Accordingly, an electronic ballast circuit is disclosed for igniting, supplying and dimming a gas discharge lamp which comprises an oscillation control circuit, an IR2155 self-oscillating half-bridge driver, a dimming control circuit, a load circuit including at least one gas discharge lamp and an over-current protection circuit, wherein the oscillation control circuit controls a current through the lamp via the pulse width modulation of a drive signal during a pre-heating stage and normal operation for the undimming ballast and the dimming ballast.
In a specific embodiment of the present invention, there is provided an electronic ballast circuit for igniting, supplying and dimming a gas discharge lamp comprising: first and second input terminals (A and B) for connection to a source of DC voltage; a controlled semiconductor switching element having a drain electrode, a source electrode and a control electrode; a IR2155 self-oscillating half-bridge driver having an oscillator timing resistor input RT, an oscillator timing capacitor input CT, a high side floating supply VB, a high side gate drive output HO, a high side floating supply return VS, a low side supply VCC, a low side gate drive output LO and a low side return GND; a choke transformer having a primary winding and a secondary winding; first means for connecting first (Q1) and second (Q2) semiconductor switching elements in a first series circuit across said first and second terminals; second means for connecting one end of a load circuit and one end of a snubber circuit to a junction point between said first and second semiconductor switching elements and further connecting the other end of said load circuit to said second terminal (B) and the other end of said snubber circuit to said first terminal (A); said load circuit comprising a first capacitor, a gas discharge lamp and a choke transformer; said snubber circuit having a first resistor and a second capacitor connected in series; third means for connecting the input terminals of a first DC supply circuit across said first and second terminals; said first DC supply circuit having a second resistor and a third capacitor connected in series; fourth means for connecting said VCC pin of the IR2155 self-oscillating half-bridge driver to a junction point of said second resistor and said third capacitor of said first DC supply via a first diode, and then to said second terminal via a fourth capacitor; and further connecting said GND pin of the IR2155 self-oscillating half-bridge driver to said second terminal; fifth means for connecting said VB pin of the IR2155 self-oscillating half-bridge driver to said VCC pin of the IR2155 self-oscillating half-bridge driver via a second diode, and further connecting said VS pin of the IR2155 self-oscillating half-bridge driver to a junction point between said first and second semiconductor switching elements, and then connecting a fifth capacitor across said VB pin and said VS pin of the IR2155 self-oscillating half-bridge driver; sixth means for connecting said HO pin of the IR2155 self-oscillating half-bridge driver to a control electrode of said first semiconductor switching element (Q1) via a third resistor, and further connecting said L0 pin of the IR2155 self-oscillating half-bridge driver to a control electrode of said second semiconductor switching element (Q2) via a fourth resistor; seventh means for connecting one end of a second DC supply to said VCC pin of the IR2155 self-oscillating half-bridge driver, and further connecting the other end of said second DC supply to said second terminal; said second DC supply comprising a secondary winding of said choke transformer, a third diode, a fourth diode and a fifth resistor connecting in series, and a sixth capacitor across one end of said secondary winding and a cathode of said third diode; eighth means for connecting a resistive voltage divider between said VCC pin of the IR2155 self-oscillating half-bridge driver and said second terminal; said resistive voltage divider having sixth and seventh resistors connected in series, and a seventh capacitor in parallel with said seventh resistor; nineth means for connecting a first part of an oscillation control circuit to said RT pin of the IR2155 self-oscillating half-bridge driver, and then connecting a second part of said oscillation control circuit to said CT pin of the IR2155 self-oscillating half-bridge driver, and then connecting a third part of said oscillation control circuit to said second terminal, and further connecting a fourth part of said oscillation control circuit to a junction point of said resistive voltage divider; said oscillation control circuit comprising a pulse-width limiting circuit, a pulse-width control circuit and an oscillation circuit; said pulse-width limiting circuit having an eighth resistor and a fifth diode connected in series and then connected in parallel with a ninth resistor; said pulse-width control circuit having a sixth diode, a third semiconductor switching element (Q3) and a tenth resistor connected in series and then connected to an eleventh resistor in parallel with an emitter electrode and a collector electrode of said third switching element; said oscillation circuit having a twelfth resistor and an eighth capacitor connected in series; tenth means for connecting one end of said pulse-width limiting circuit to said first part of said oscillation control circuit, and then connecting the other end of said pulse-width limiting circuit to one end of said pulse-width control circuit and one end of said oscillation circuit, and then connecting a collector electrode of said third semiconductor switching element to a junction point of said oscillation circuit and to the second part of said oscillation control circuit, and then connecting the other end of said pulse-width control circuit and the other end of said oscillation circuit together to the third part of said oscillation control circuit, and further connecting a base electrode of said third semiconductor element to the fourth part of said oscillation control circuit; eleventh means for connecting one end of a dimming control circuit to a junction point of said resistive voltage divider, and then connecting the other end of said dimming control circuit to said second terminal; said dimming control circuit comprising a thirteenth resistor and an opto-transistor OP1 connected in series, a ninth capacitor across the base electrode and emitter electrode of said opto-transistor, and a fourteen resistor connected in series with an opto-diode of said opto-transistor; twelfth means for connecting one end of a protection control circuit to a junction point of said first DC supply, and connecting the other end of said protection control circuit to said second terminal; said protection control circuit having a fifteenth resistor, a silicon controlled rectifier and a sixteenth resistor connected in series, and a tenth capacitor connected in parallel with the sixteenth resistor and a fourth semiconductor switching element (Q4) with a base electrode connected to a junction point of said sixteenth resistor and said silicon controlled rectifier, and a collector electrode connected to the base electrode of said third semiconductor switching element; thirteenth means for connecting one end of a current sensing circuit to a control electrode of said silicon controlled rectifier and other end to a junction point of said third diode and said fourth diode; said current sensing circuit comprising a seventeenth resistor, a zener diode and an eighteenth resistor connected in series, and an eleventh capacitor connected in parallel with said seventeenth resistor and a twelfth capacitor connected between a junction point of said zener diode and said eighteenth resistor.
An oscillation control circuit of the electronic ballast circuit embodying the present invention controls the pulse-width of the drive signal through the third semiconductor switching element (Q3). The off-time of drive signal is fixed through the ninth resistor and twelfth resistor and the eighth capacitor. The tenth resistor is placed in parallel with the eighth capacitor is to control the off-time near the fixed value during the starting stage. The on-time of the drive signal can be varied by the third semiconductor switching element (Q3) and the sixth diode by setting the base current of Q3 through the resistive voltage divider. The value of the sixth resistor and seventh capacitor in the resistive voltage divider determines the time for the preheating during the starting stage. The eighth resistor and the fifth diode control the minimum pulse-width of the drive signal when the third semiconductor switching element is almost fully on. The eleventh resistor across the third semiconductor switching element controls the maximum pulse-width of the drive signal when the third semiconductor switching element is almost fully off. The oscillation control circuit is able to provide the light output control of the lamp via the pulse-width modulation of the drive signal.
An embodiment of the present invention can control the light output over a wide range through the dimming control circuit comprising the opto-transistor and the control signal input terminals of said opto-transistor. The input voltage across said control signal input terminals of the opto-transistor control the base current of the opto-transistor which controls the base current of the third semiconductor switching element, and then modulates the pulse-width of the drive signal. Using opto-transistor, the dimming control circuit can easily be implemented by providing the voltage across the control signal input terminals of the opto-transistor for remote control.
An embodiment of the present invention is based on the recognition that upon switching on the ballast the third capacitor of the first DC supply circuit is first charged until the voltage on the IR2155 self-oscillating half-bridge driver is about 5 volts to provide the initial oscillation of the oscillation control circuit. As a result, the voltage divider sets the minimum pulse-width of the driver signal and the current starts to flow through the gas discharge lamp filament and heat it up. Whilst the voltage of the secondary winding of the choke transformer is being built up, the second DC supply is taken over by the first DC power supply and powers the IR2155 self-oscillating half-bridge driver and the control circuit. The supply voltage for the IR2155 self-oscillating half-bridge driver and the control circuit is gradually increased to the operating voltage of 15 volts. Meanwhile, the pulse-width of the drive signal is gradually increased up to the preset value. The lamp starts to ignite and the output power for the lamp is gradually increased to the preset level. The output power for the lamp can be set by tuning the voltage divider. The maximum output voltage of the first DC supply is set to 5 volts by tuning the resistance value of the second resistor of the first DC supply so as to reduce its power dissipation and to provide soft start and preheating functions. The preheating time can be adjusted by tuning the sixth resistor and seventh capacitor. During normal operation, the brightness of the lamp can be tuned from 0% to 100% by providing 10 volts to 0 volt to the control signal input terminals of the dimming control circuit.
According to an embodiment of the present invention, the over-current protection circuit comprises at least a silicon controlled rectifier and a zener diode connected to the secondary winding of the choke transformer that is shared with the second DC supply. The secondary winding of the choke transformer is used to supply the power for the driver circuit and control circuit and to sense the output current. When the output current increases to the maximum value and the voltage of the secondary winding across the zener diode reaches its breakdown voltage, the zener diode starts to conduct and charges up the eleventh capacitor. As soon as the voltage across the eleventh capacitor reaches a firing voltage of the silicon controlled rectifier, the silicon controlled rectifier conducts and provides the large base current to turn on the fourth semiconductor switching element. As a result, the third semiconductor switching element is fully turned on and the oscillation of the oscillation control circuit stops. The protection circuit is latched until the ballast is reset. The maximum current value of the ballast can be set by selecting the breakdown voltage of the zener diode.
The invention is particular advantageous for use in low-pressure mercury vapour discharge lamps where the heating filament is used to ignite the lamp. Lamp life is dependent upon the preheating control during ignition. An embodiment of the present invention makes it easy to control the preheating time by varying the capacitance value of the seventh capacitor. This offers an extension of the lamp life.
An advantage of one embodiment of the ballast circuit is that the lamp can dimmed by modulating the pulse-width of the drive signal for local control and remote control. The protection circuit provides an easy way to expand current protection to full protection, i.e. input over-voltage protection, by triggering the silicon controlled rectifier.
In order that the invention may be more readily understood, and so that further features thereof may be appreciated, an embodiment of the present invention will now be described with reference to the accompanying drawing which illustrates diagrammatically an embodiment of the electronic ballast circuit according to the present invention.